Thermochemical Modeling of Laves Phase Containing Ferritic Steels

Jacob, Aurélie

Dissertation / PhD Thesis/Book

FZJ-2015-05316

Thermochemical Modeling of Laves Phase Containing Ferritic Steels

Jacob, A. (Corresponding author)FZJ*

2015
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 978-3-95806-070-8

Jülich : Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag, Schriften des Forschungszentrums Jülich Reihe Energie & Umwelt / Energy & Environment 274, 200 S. (2015) = RWTH Aachen, Diss., 2015

Please use a persistent id in citations: http://hdl.handle.net/2128/9203

Abstract: For energy technology, development of new materials is required to be used for boiler and steam turbineswith high thermal cyclability, creep strength and excellent corrosion resistance in steam and high temperatures and also at downtime in water. Crofer 22H, a new kind of steel jointly developed at Forschungszentrum Juelich and Thyssen Krupp, has already shown promising results in laboratory tests. The formation of Laves phase particles improves the material’s properties such as creep strength while the high Cr-content of 22 % provides excellent corrosion resistance. Unfortunately, the formation of Laves phase competes with the formation of other phases. In order to further improve this type of ferritic steels, knowledge of thermodynamics of the intermetallic systems forming the Laves phases(Fe,Cr,Si)$_{2}$(Nb,W) is required for the calculation of phase stability as a function of composition and temperature. The Calphad method is of great help for the calculation of phase stability in technical materials. This kind of calculation is based on thermodynamic databases. Calculations with the commercial steel database have shown to be inaccurate since not all considered alloy systems have been modeled up to now. The present work aims to provide a thermodynamic optimization of the systems containing Laves phases. Thermodynamic data of Laves phase containing systems were obtained by the Calphad method with an optimization of the sub-systems containing Laves phases. The Calphad method relies on the input data for the considered system, their quality and their quantity are of primary importance for the obtained thermodynamic assessment. In the present work, several methods have been used in order to provide consistent thermodynamic calculations of the phase equilibria. A combined experimental and modeling approach was chosen to determine the phase equilibria and thermodynamic properties in the systems Fe-W, Cr-Nb, Cr-Fe-Nb and Fe-Nb-Si. DFT calculations were done for all mentioned systems for the determination of the energy of formation of the compounds through the chosen model. In the Fe-W system, in addition to the DFT calculations which are restricted to 0 K, phonon calculations were performed in order to obtain the temperature dependence of the Gibbs energies which were used to improve the Calphad modeling.

Note: RWTH Aachen, Diss., 2015

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